Electronic components are often transported in (JEDEC) trays (a molded matrix carrier) during the manufacturing process. A conventional JEDEC tray is shown in FIG. 1. The components are placed in the cells of the trays, and placing components in cells is usually done by pick and place robots. A pick and place robot typically comprises one or several grippers (or pickers) that can pick-up/remove an electronic component from a cell of a tray and also place an electronic component into a cell of a tray. The gripper may be mechanically clamping the electronic component or—more commonly—using vacuum nozzles.
For example, U.S. Pat. No. 8,903,541 B2 discloses a method for positioning electronic devices into compartments. Known positions on an upper side of the input medium are imaged by an inspection device. Actual position data of the compartments of the input medium are calculated on the basis of images of the known positions, given target position data of the known positions, and given target position data of the compartments of the input medium. A pick and place device for the electronic devices is controlled on the basis of the calculated actual position data of the compartments of the input medium.
The international patent application WO 2014/143436 A1 relates to an apparatus for automatic pitch conversion for pick and place heads, comprising at least one auto pitch station for adjusting a pitch in a X coordinate direction and/or adjusting a pitch in a Y coordinate direction of pickers/grippers of a pick and place heads. The pick and place device has a sensing system, which is used to verify the position and the pitch of the pickers/grippers of the individual pick and place heads. Calibration is done as well by the sensing system. There is nothing disclosed, that a verification is carried out of the distance between the pickers/grippers and the electronic component.
Additionally, EP 2 699 071 A2 discloses a head module to pick up and place dedicated components in SMT technology. A body of a head module has a suction nozzle assembly with a gripping element comprising a vacuum duct inside the gripping element. At one end the gripping element is equipped with suction nozzle dedicated to grip components. Vacuum or negative pressure supplied to the suction nozzle is changed using the stepwise method by application of a cut-off valve, thus the suction nozzle sucks and holds the electronic component only when vacuum or negative pressure is supplied to it. After cutting off the vacuum or the negative pressure, the suction nozzle releases and leaves the electronic components in the predetermined place. A linear movement of the gripping component (end position of the gripping component) with the suction nozzle is controlled with an optical switch 98.
A most common method is that the user estimates the contact point visually and adjusts the pick-up height accordingly. Another possibility is the use of the vacuum supply and pressure sensor to detect contact between a picker and an object. This method is used to detect contact between the picker and a heavy object, such as a metal calibration tray or a fixed part of the machine. The picker (with/or without adapter) is moved to the metal part. The picker moves (with vacuum on) down towards the calibration tray. The pressure sensor detects when the vacuum level suddenly builds up in case of contact.
The visual estimation often leads to inaccurate positions due to difficult accessibility of the pickers. Mostly the pickers are close to the devices and the assembly that carries the pickers blocks the visibility. In most cases, safety doors and other assemblies are blocking light to access the area where the visual check is needed. Assessing the correct contact point visually depends on the human capabilities (vision accuracy) so larger errors are common. Using the vacuum supply leads to wrong detection points in case of medium and small devices.
The devices can be sucked up by the vacuum, leading to a wrong detection position of the contact.
When using a vision measurement system, the relative height difference between the measured object and the moveable picker height has to be calibrated or measured. This is very hard to calibrate since the pickup adapter can differ in height after a changeover. In addition, using a displacement sensor or any other vision system is detrimental to the cost effectiveness.